Automotive steering devices typically consist of a rack and pinion mechanism as illustrated in FIG. 6. As well known in the art, a steering wheel 1 is securely attached to an upper end of a steering shaft 2, and a lower end of the steering shaft 2 is connected to a coupling shaft 4 via a pair of universal joints 3. A pinion 5 is securely attached to a lower end of the coupling shaft 4, and meshes with a rack 6 which is connected to road wheels 9 via tie rods 7 and knuckle arms 8 as well known in the art. Thus, the rotational movement (.beta.) of the pinion 5 is converted into a linear movement (L) of the rack 6 meshing with the pinion 5, which is in turn converted into a steering movement (T) of the road wheels 9 via the tie rods 7 and the knuckle arms 8.
According to such conventional steering devices, the steering angle (T) of the road wheels 9 changes substantially linearly with the rotational angle (.alpha.) of the steering wheel 1. However, in view of the handling of the vehicle, the rotational angle of the steering wheel 1 for achieving a maximum steering angle of the road wheels 9 is desired to be fairly small. More specifically, if the relationship between the rotational angle of the steering wheel and the steering angle of the road wheels is determined as indicated by a chain dot line (v) of FIG. 7, the necessary rotational angle of the steering wheel 1 is reduced, and a favorable vehicle handling can be achieved in a low speed range, but the vehicle response (yaw rate, lateral acceleration and rolling movement) may become so excessive in a high speed range that the vehicle handling in the high speed range may not be acceptable. It means that any fixed linear relationship between the rotational angle of the steering wheel and the steering angle of the road wheels cannot be satisfactory in all speed ranges.
Based on these considerations, the relationship between the rotational angle of the steering wheel and the steering angle of the road wheels is normally determined according to the compromise between the desired vehicle responses in both high and low speed ranges, and is typically determined in such a manner that a maximum steering angle can be achieved by turning the steering wheel 1.5 turns or 540 degrees from a neutral position in either direction as indicated by the double chain dot line (w) in the graph of FIG. 7.
The graph of FIG. 8 conceptually shows the changes in the maximum practical steering angle with the vehicle speed. The fine double chain dot line (x) indicates the maximum practical steering angle for the conventional steering system. When the vehicle speed is extremely low, the steering wheel can be turned by up to 1.5 turns. However, in a high vehicle speed range, the rotational angle of the steering angle must be kept extremely small in order to operate the vehicle in a stable fashion. Ideally, it is desired to reduce the maximum practical steering wheel angle to a lower level in a low speed range, and to increase it in a high speed range as indicated by the bold curve (y) in FIG. 8. This can be accomplished by reducing the gear ratio between the steering wheel and the road wheels (.alpha./.beta.) in the low speed range and increasing it in the high speed range.
The variable steering angle ratio steering system proposed in copending U.S. patent application Ser. No. 08/192,577 filed Feb. 7, 1994 now U.S. Pat. No. 5,386,899 was intended to achieve such a goal. As illustrated in FIG. 11 of this application, the system proposed in the copending patent application comprises an input shaft 101 connected to the steering wheel, and rotatably supported by a substantially cylindrical support member 103, via a dual radial beating 102, which is in turn slidably supported by a casing 106 via support pins 104 and 105 integrally provided in the support member 103. An eccentric shaft 108 which is integrally provided in an output shaft 107 is coupled to the input shaft 101 via a slide coupling 109. The output shaft 107 is coupled to a pinion 5 which in turn meshes with a rack 6 for transmitting a steering torque to the road wheels. According to this steering system, by moving the support member 103 perpendicularly to the input shaft 101 and changing the eccentricity between the input shaft 101 and the output shaft 107, the steering angle ratio can be changed in a continuous manner.
According to this structure, however, if the mechanical rigidity of the power transmitting members of the steering system is not sufficient and/or the fabrication precision is not sufficient, the steering angle ratio may be affected by the magnitude of the steering load, and an unacceptable play may be created in the steering system. Also, the process of varying the steering gear ratio may not be smoothly carried out if the fabrication precision is not fairly high.
In particular, because this previously proposed variable steering ratio steering system involves a relatively low steering gear ratio as opposed to the conventional steering system based on the compromise between the requirements of both high and low speed ranges, it is necessary to increase the rigidity and make the gear ratio varying mechanism operate all the more smoothly. However, because the support pins 104 and 105 are slidably engaged by the casing 106, it is difficult to ensure a sufficient dimensional precision, to achieve a high supporting rigidity and to allow the gear ratio varying mechanism to operate smoothly.
Another variable ratio steering system is proposed in copending U.S. patent application Ser. No. 08/191,659 filed Feb. 4, 1994. The contents of these copending patent applications are incorporated herein by reference.
Japanese patent laid open publication (kokai) No. 2-14971 discloses a steering device using a pair of elliptic gears and another pair of circular gears between the input shaft and the output shaft so that the gear ratio may be reduced as the steering input is increased. As a result, the angular displacement of the pinion is relatively small for a given increment of the steering input when the steering input is small, but the angular displacement of the pinion becomes larger for the same increment of the steering input when the steering input is large. Thus, it is possible to achieve a steering property as indicated by the curve (y) in FIG. 8, and the handling of the vehicle steering device remains substantially uniform over the entire speed range of the vehicle.
However, machining elliptic gears requires a special tool arrangement, and achieving a necessary precision is relatively difficult to accomplish. Also, the vehicle operator may experience some fluctuations in the effort required to turn the steering wheel, and may receive an unfavorable impression. Also, the speed reducing property of the gear mechanism of the steering device is determined by the shape of the elliptic gears, and cannot be changed in any simple manner. Accurately meshing the four gears with each other is essential for a favorable handling of the steering device, but it requires some special care. Removal of backlashes from the gear train including both elliptic gears and circular gears is difficult, and some play is inevitable in the gear train of the steering device from a practical view point.